how can i keyframe on animation node properties?
you cannot animate the falloff because the falloff basically is just an array of numbers.
I think, what you wanna do is:

The timeinfo just outputs the frame number (i think you know that) and the fade falloff recalculates this to values between 0 and 10, where amount is just the step value. Of course you can use whatever falloff node you want here. So the falloff will be something like [0.1, 0.2, 0.3 ...] as you can see in the viewer.
The evaluate falloff node then just gives you the value at the index of that array.
you can try out itself in this easy example, which just moves a cube: